That 1099 for income received in 2018 belongs on your 2018 tax return. You cannot add it to a 2019 return if you are asking how to do that. If you filed a 2018 return without that income then you need to amend the 2018 return. If you (he) did not file a 2018 return at all then he needs to file a 2018 return to report that income.
You are posting from TurboTax Business for Windows--which cannot be used for a personal income tax return. What are you trying to do?
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